High reflectance, high heat dissipation, dielectric strength, and long-term reliability may be raised as characteristics that a substrate used for light emitting devices has to basically have. Particularly, a substrate for light emitting devices that is used for high luminance illumination has to have high dielectric strength.
In related art, as a substrate for light emitting devices, a ceramic substrate, a substrate that includes an organic resist layer as an insulating layer on a metal substrate body, or the like has been known. A description will be made about configurations of a ceramic substrate and a substrate that uses a metal substrate body.
(Ceramic Substrate)
For example, a ceramic substrate is fabricated by forming an electrode pattern on a plate-shaped ceramic substrate body. Accompanying the tendency of the output increase of light emitting devices, the improvement in brightness has been pursued by arranging a large number of light emitting elements on a substrate. As a result, the size of ceramic substrates has been increasing every year.
Specifically, in a case where a common LED light emitting device used at input power of 30 W is realized by arranging blue LED elements with dimensions of approximately 650 μm×650 μm or dimensions around those on one substrate that is categorized into a medium size, for example, approximately 100 blue LED elements are necessary. One example of the ceramic substrate in which the approximately 100 blue LED elements are arranged is a ceramic substrate in a plane size of 20 mm×20 mm or larger and with a thickness of approximately 1 mm.
Further, in a case where an attempt is made to realize a brighter LED light emitting device at input power of 100 W or more, technical development based on such a size increase of the ceramic substrate results in the necessity of a larger ceramic substrate at least in a plane size of 40 mm×40 mm or larger, on which 400 or more blue LED elements may together be installed.
However, even if an attempt is made to realize the ceramic substrate in an increased size on a commercial basis in response to the demand of the above-described size increase of the ceramic substrate, the realization on a commercial basis has been difficult due to three problems of the strength of the ceramic substrate, the production accuracy, and the producing cost.
Specifically, because a ceramic material is basically porcelain, a problem of strength occurs to the ceramic substrate in an increased size. In a case where the ceramic substrate is thicken to overcome the problem, new problems occur that the thermal resistance increases (the heat dissipation degrades) and at the same time the material cost of the ceramic substrate rises. Further, in a case where the size of the ceramic substrate is increased, not only the external dimensions of the ceramic substrate but also the dimensions of the electrode pattern formed on the ceramic substrate are likely to become inaccurate. This results in a problem that the production yield of the ceramic substrate decreases and the production cost of the ceramic substrate is likely to rise.
(Substrate Using Metal Substrate Body)
Further, for example, there is a case where a highly thermally conductive metal substrate body is used as a substrate that is used for a high-output light emitting device for the purpose of overcoming the above problems with the ceramic substrate. Here, in order to install a light emitting element on a metal substrate body, an insulating layer has to be provided on the metal substrate body to form an electrode pattern to be connected with the light emitting element. Further, the insulating layer has to have high light reflectivity in order to improve the light use efficiency in a substrate for high-output light emitting devices.
An organic resist is raised as a material used as the insulating layer in related art in the substrate for high-output light emitting devices. Further, a layer that serves as both of light reflection layer and insulating layer may be formed by using ceramic-based paint.
In a case of using an organic resist that is used as the insulating layer in related art in the substrate for high-output light emitting devices, sufficient thermal conduction, heat resistance, or light resistance may not be obtained, and dielectric strength necessary for the substrate for high-output light emitting devices may not be obtained either. Further, light that leaks toward the metal substrate body side via the insulating layer has to be reflected in order to improve the light use efficiency. However, sufficient light reflectivity may not be obtained in a configuration that uses an organic resist in related art as the insulating layer.
Differently, in a case where the layer that serves as both of light reflection layer and insulating layer is formed on the metal substrate body in the substrate for high-output light emitting devices by using ceramic-based paint, a substrate for light emitting devices that has proper reflectance, heat resistance, and light resistance may be realized. PTL 1 discloses a formation method of a layer that serves as both of light reflection layer and insulating layer, in which ceramic-based paint is coated onto a substrate body.
Further, although not limited to paint, for example, PTLs 2 to 4 disclose substrates for light emitting devices in which the layer that serves as both of light reflection layer and insulating layer and is formed of ceramics is formed on a metal substrate body by coating or spraying.